The Puppet Masters — Robert A. Heinlein

DEADLY EXPOSURE
At key points throughout North America, an invasion force is taking over communications, government, industry--and people's bodies. And the nation is helpless to stop it because the invaders multiply far faster than they can be destroyed, controlling the mind of every unsuspecting person they encounter....

Enter Sam Cavanaugh, a can-do intelligence officer for the United States' most secret service. Cavanaugh is the only man who can stop the invaders. But to do that he'll have to be invaded himself!